Straightening or relaxing the curls of very curly hair may increase the manageability and ease of styling of such hair. In today's market, there is an increasing demand for the hair care products referred to as “hair relaxers,” which can relax or straighten naturally curly or kinky hair. Hair relaxers may either be applied in a hair salon by a professional or in the home by the individual consumer.
Hair fiber, a keratinous material, comprises proteins (polypeptides). Many of the polypeptides in hair fibers are bonded together or cross-linked with disulfide bonds (—S—S—). A disulfide bond may be formed from the reaction of two sulfhydryl groups (—SH), one on each of two cysteine residues, which results in the formation of a cystine residue. A cystine residue comprises a cross-link of the formula —CH2—S—S—CH2— between 2 polypeptides. While there are other types of bonds which occur between the polypeptides in hair fibers, such as ionic (salt) bonds, the permanent curling or the shape of the hair is essentially dependent on the disulfide bonds of cystine residues.
Generally, hair relaxing processes are chemical processes which may alter the aforementioned disulfide bonds between polypeptides in hair fibers and may form lanthionine residues [S[CH2CH(NH—)(CO—)]2]. Thus, the term “lanthionizing” is used when one skilled in the art refers to the relaxing of keratin fibers by hydroxide ions. “Lanthionizing,” as used herein, refers to the formation of at least one lanthionine residue, which may accomplish, for example, any level of relaxation. “Relaxation” and “relaxing,” as used herein, includes any level of relaxing, for example, from slight relaxing to straightening.
For example, hair fibers may be relaxed or straightened by disrupting the disulfide bonds of the hair fibers with an alkaline reducing agent. The chemical disruption of disulfide bonds with such an agent is generally combined with mechanical straightening of the hair, such as combing, and straightening generally occurs due to changes in the relative positions of neighboring polypeptide chains within the hair fiber. This reaction is generally terminated by rinsing and/or application of a neutralizing composition.
The reaction with the alkaline agent is normally initiated by available hydroxide ions. As used herein, “available hydroxide ions” are hydroxide ions which are available for lanthionization. Not to be limited by theory, there are two reaction sequences that are predominantly used in the art to explain the disruption of the disulfide bonds in hair fibers by available hydroxide ions. Both of these reaction sequences result in lanthionine residue formation. Generally, hydroxide ions initiate a reaction in which a cystine cross-link (—CH2—S—S—CH2—) is broken and a lanthionine cross-link (—CH2—S—CH2—) is formed. The lanthionine cross-link is shorter than a cystine cross-link by one sulfur atom, and thus the net effect of the reaction is to reduce the distance between polypeptides. Amino acid analysis indicates that from 25 mole % to 40 mole % of cystine residues are converted to lanthionine residues.
One reaction sequence comprises at least one bimolecular nucleophilic substitution reaction wherein an available hydroxide ion directly attacks the disulfide linkage of a cystine residue. The result is the formation of lanthionine residues and HOS−. See Zviak, C., The Science of Hair Care, pp. 185-186 (1986). The second reaction sequence comprises at least one β-elimination reaction initiated by the nucleophilic attack of an available hydroxide ion on a hydrogen atom bonded to a carbon atom that is in the p-position with respect to the disulfide bond of a cystine residue. Id. The result is the formation of a dehydroalanine residue which comprises a reactive double bond (═CH2). The double bond of the dehydroalanine residue can then react with the thiol group of a cysteine residue to form a lanthionine residue. These stable irreversible crosslinks in the treated hair make subsequent chemical re-linking of the polypeptides unnecessary. Thus, the only step that may be required following a straightening process using such hydroxide-containing alkaline agents is the removal of any excess alkaline solution to avoid or minimize damage to the hair protein or skin. If such a step is required, an acidic shampoo may be used to neutralize residual alkali and remove it from the hair and scalp.
Hydroxide-containing alkaline agents also have other advantages. For example, alkaline agents, such as sodium hydroxide and guanidine hydroxide, do not have a highly objectionable odor or cause such an odor on treating the hair. Further, hydroxide-based straighteners generally have relatively fast processing times and good straightening of naturally curly or kinky hair. Additionally, the achieved straightening effect is more durable; i.e., less likely to revert to a curly state after shampooing and exposure to the elements than is hair straightened with some other straighteners.
Despite these advantages, certain hydroxide-containing alkaline agents may have disadvantages. These disadvantages may be heightened when the hydroxide-containing alkaline agent is sodium hydroxide. Specifically, the causticity of sodium hydroxide can adversely affect the condition of the hair, for example, leaving it in a brittle state and harsh to the touch. Additionally, prolonged or unnecessary exposure of hair to such a strong alkali can weaken, break and dissolve the hair. The mechanical properties of hair that has been lanthionized using hydroxide ion generating compositions demonstrate that, while the hair may not be significantly weaker due to the reduction in space between polypeptides (and in fact may have a high yield force), the hair may have a lower elongation before breaking. This “brittleness” of high yield force coupled with low elongation and inherently weaker points (where the hair had natural twists) can lead to breakage during grooming. Further, in some instances, such a strong alkali can discolor the natural color of the hair. For example, the tone of natural brown hair may be reddened and natural white or grey hair may be yellowed. Further, the natural sheen of the hair may be delustered.
Most frequently, commercial relaxing compositions are in the form of gels or emulsions that contain varying proportions of strong water-soluble bases, such as sodium hydroxide (NaOH), or of compositions that contain slightly-soluble metal hydroxides, such as calcium hydroxide (Ca(OH)2), which can be converted in situ to soluble bases, such as guanidine hydroxide. Traditionally, the two main hair relaxers used in the hair care industry for generating hydroxide ions are referred to as “lye” (lye=sodium hydroxide) relaxers and “no lye” relaxers.
The “lye” relaxers generally comprise sodium hydroxide in a concentration ranging from 1.5% to 2.5% by weight relative to the total weight of the composition (0.38M-0.63 M) depending on the carrier used, the condition of the hair fibers and the desired length of time for the relaxation process.
While “no lye” relaxers may not contain lye, they may, however, rely on the soluble hydroxides of inorganic metals, such as potassium hydroxide and lithium hydroxide. Other “no lye” relaxers may use hydroxide ions obtained, for example, from a slightly-soluble source, such as Ca(OH)2. For example, the slightly soluble Ca(OH)2 may be mixed with guanidine carbonate to form guanidine hydroxide, a soluble but unstable source of hydroxide, and insoluble calcium carbonate (CaCO3). This reaction is driven to completion by the precipitation of CaCO3 and is, in effect, substituting one insoluble calcium salt for a slightly soluble calcium salt. Because guanidine hydroxide is unstable, the components are stored separately until the time of their use.
Some strides have been made to improve the condition of sodium hydroxide-straightened hair by incorporating an auxiliary hair keratin disulfide reducing agent having a sulfhydryl functional group available and chosen from cysteine, homologs of cysteine, and water soluble derivatives of cysteine. See, for example, U.S. Pat. No. 4,992,267, the disclosure of which is incorporated herein by reference. This patent discloses the use of sodium hydroxide at concentrations of between about 1 weight percent to about 2.5 weight percent, preferably between about 1.5 weight percent and about 2.25 weight percent relative to the total concentration of the composition.
Further, co-pending U.S. patent application Ser. No. 09/789,667, the disclosure of which is incorporated herein by reference, discloses compositions, and methods for using these compositions, for lanthionizing keratinous fibers comprising at least one hydroxide compound with the proviso that the at least one hydroxide compound is not sodium hydroxide, lithium hydroxide or potassium hydroxide and at least one activating agent chosen from cysteine-based compounds. These compositions may make it possible to even further decrease the amount of the at least one hydroxide compound needed while maintaining good hair condition.